National Pollutant Discharge Elimination System

Under the Clean Water Act of 1972, water pollution control facilities in the U.S. are required to have an NPDES permit in order to discharge water to waterways.

NPDES permits regulate how much flow can be discharged to a receiving stream and set limits on pollutants such as biochemical oxygen demand, total suspended solids, and ammonia nitrogen.The Ames facility [shown here] was issued a permit by the Iowa Department of National Resources (DNR), with approval from the EPA. It is allowed to discharge 21 million gallons per day (MGD) but averages 6.

Shown in this picture: wastewater with ammonia and organic matter is pumped into the top of a trickle filter. It passes through a plastic matrix composed of corrugated plastic sheets and hollow cylinders lined with biofilm. These microbes reduce the biochemical oxygen demand and ammonia before it is discharged into the Skunk River.